1. Field of the Invention
The present invention relates to separation of a mixture comprising water, a fluid not miscible with water and having a lower density than water, and a gas into these constituents.
In particular the present invention relates to a combined degassing and flotation tank, which is particularly suited for use in separation processes where a water phase containing oil and gas is separated into these constituents.
The invention is applicable within a broad range of industries comprising the petroleum industry, including oil exploration and production, refining, transport, storage, cleaning of tanks and/or equipment etc.
2. Description of the Prior Art
In the oil industry the production of crude oil involves pumping a mixture of oil, gas and water from subterranean reservoirs. At the well-head a initial separation usually takes place, and the crude oil still containing some gas and water is treated in one or more separators to remove more water and gas before the crude oil is ready for refining.
At offshore oil production the water phase coming from the separation at the well-head or subsequent separators is usually discharged into the sea after a cleansing that involves the partial removal of gas, oil, chemicals and other impurities. Today this cleansing is accomplished using large equipment such as oil/gas separators, flotation tanks, hydro cyclones and degassing tanks occupying a substantial space at the production platforms.
With aging of the oil fields, one often finds that the volume of water accompanying the oil becomes much larger and consequently the capacity for water treatment often needs to be increased to treat the increasing amounts of water.
Further, there is a general concern of the pollution caused by oil production at sea, particularly when the oil production takes place in areas that are considered to be environmentally fragile, such as arctic areas or fishing areas. In the oil industry there is a fear that a demand for a significantly lower limit for the outlet of oil would make oil production from a number of the known reservoirs to be economically unfeasible if one has to rely on the equipment used at present. Thus great efforts have been made by industry and authorities to find ways to reduce the outlet of oil during oil production at affordable prices.
On oil production platforms intended for operation at sea, very limited space is usually available. Therefore there are very strict constraints on the space available for installation of new equipment that may be needed in order to handle increasing amounts of water, in particular if one should honor the rising concerns for the environment and reduce the pollution. An even stricter constraint on space may be encountered if one considers the establishment of oil production at the sea bed level.
In the prior art a number of oil-gas-water separators are known. In U.S. Pat. No. 4,424,068, a separator and a method for separating a mixture of oil, gas and water, such as may be received from an oil well, is described. The separator is in the form of a vessel divided into separation chambers and provided with a number of baffles and a dynamic separator where the incoming mixture changes direction several times. Despite the fact that this separator has been known for several years it seems not to have been widely used. Further as the separator comprises several chambers and many parts, the maintenance will be time consuming which may lead to costly stop of oil production.
U.S. Pat. No. 4,364,833 describes a separator comprising one sector containing numerous plates arranged horizontally and parallel in order to confer coalescence of small oil drops to larger oil drops, and a second sector containing numerous plates arranged vertically and parallel in order to let the formed larger oil drops rise to the top where they form an oil layer which is collected. As the separator contains a number of narrow passages between the parallel plates, these passages are susceptible to clogging by solids contained in the incoming flow, which may lead to the need to stop for cleaning.
U.S. Pat. No. 3,797,203 describes a separator containing a number of conically formed screens on the surface of which small oil droplets coalesce to larger oil drops which rise to the top of the separator where they can be collected as an oil phase.
WO 99/20873 describes a sand trap that may be placed on an oil well in order to remove heavier particles such as sand before further processing of the crude oil. The device has a mouth towards a relatively narrow part of a tank with a spatial connection towards a relatively widened part of the tank where sand and heavy particles precipitate.
Despite the number of known oil-gas-water separators, there is still a need for an improved oil-gas-water separator with a better separation of the phases, which does not need a large space, which can be continuously operated with low requirements for maintenance and which can be manufactured and operated at moderate cost.